Frequency modulated altimeter or distance indicator



June 10 1947 R. c. SANDERS, JR 2,42233 FREQUENCY MODULATED ALTIMETER CR DISTANCE INDICATOR Filed Dec. 30, 1942 2 Sheets-Sheet l June 10, 1947- R. c. SANDERS, JR 2 2,422,133

FREQUENCY MODULATEDRALTIMETER OR DISTANCE INDICATOR Suventor 1 a Patented June 10, 1947 FREQUENCY MODULATED ALTIMETER R DISTANCE INDICATOR Royden C. Sanders, Jr., Hightstown, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 30, 1942, Serial No. 470,695

6 Claims. (omai-1.68)

' My invention relates to radio systems such as radio altimeters or other distance indicating apparatus of the type utilizing reflected waves. It will be described particularly with respect to sysa change in this distance, it will not greatly aiiect the operation of the invention, providing the change is not too rapid, even though such a change causes the beat frequency signal to start at a slightly diierent part of the cycle at the V* beginning of each frequency-modulation sweep.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure l1 is a circuit diagram ci a frequency- G0 2 modulation distanceindicator designed in ac cordance with one embodiment of the invention; Figure 2 is a graph illustrating the manner in which the transmitter is cyclically frequency tems wherein the transmitted wave is cyclically '5 modulated; Figure 3 is a graph illustrating the frequency modulated. periodic or repetitive characteristic of the beat Systems of this character are described in frequency signal supplied to the audio or beat Bentley Patent 2,011,392` and Espenschied Patent frequency amplier of Fig. 1; and Figure 4 is a 2,045,071. In these systems7 the radiated freblock diagram of another embodiment of the inquency-modulated Wave is reflected from the 10 vention. In the several figures similar parts are earths surface or other object and the reflected indicated by similar reference characters. wave is received in a heterodyne receiver located Referring to Fig. v1, the apparatus comprises a in the vicinity of the transmitter. The hetercfrequency modulated radio transmitter I0 .hav dyningY or mixing signal for the receiver is obing an antenna such as a dipole l I which radiates tained directly from the transmitter whereby the a frequency modulated signal'toward the target received output includes a signal of beat freor other reflecting object. Such a signal is proquency which frequency is a function of the time duced by sweeping the frequency of the carrier required for the radiated signal to reach the rewave of the transmitter IG through a frequency ecting Yobject and return to the receiver. band at the periodic rate fm whereby a cyclically An object of the invention is to provide an imfrequency-modulated .carrier Wave is obtained. proved distance measuring system of the type rihis may be done by means of a suitable f1 utilizing reflected waves. quency modulator I2, which in one simple form A further object of the invention is to provide may comprise a motor driven capacitor for varyan improved method of and means for increasing ing the tuning of the transmitter oscillator as the signal-to-noise ratio of the beat frequency described in the above-mentioned Espenschied signal output ci a receiver in a frequency-modupatent, or it may comprise a vibrating capacitor lated distance determining system. or a reactance tube circuit. In a preferred embodiment of the invention, The specific receiver, which will be described advantage is taken of the fact that the beat irefor the purpose of illustration, includes a heteroquency wave pattern of the receiver output redyne detector I5 comprising two vacuum tubes peats at the modulating frequency fm, that is, at I3 and l1 connected in balanced relation. The the frequency at which the transmitted signal is reflected signal is picked up by an antenna such cyclically swept through a certain frequency as a dipole la and applied in push-pull relation band. Because of this repetitive characteristic to the input circuits of the tubes I5 and I1. The of the receiver output, it is possible to provide the heterodynlng SignalV is applied in parallel relaaUdO 0I' beat ffeqllenCy amplifier of the receiver tion from the transmitter I@ over aline I9 to the with a regenerative feedback circuit in which a input circuits of the tubes I6 and Il, this signal delay network introduces the correct amount of being the same as that radiated from antenna time delay to cause the beat frequency signal that i l, The output circuit of the balanced detector iS Dl'OduCed during 011e frequenCy-Irwdlllaton 40 includes a transformer 2| that has a centersweep to be reinforced by the beat frequency sigtapped primary 22 and a secondary 2.3'. Anode nal that was produced during the preceding frevoltage is supplied from a suitable B supply quen-Cy-moduliaton SWeep. The amount of time (not shown) through the center tap and through dOlay required fOr this iS the DelOd 0f one fleopposite halves of the primary 22 to the anodes quency-modulation sweep oratime delay equal to "15 0f tubes I6 and il; The secondary 23 supplies this period multiplied by a Whole number. S-trictthe beat frequency output signal to an audio'irely Speaking, the foregoing remarks apply Only 150 quency amplifier 24 and to a suitable frequency the case where the distance from the distance counter or indicator 25. measuring system to the reilecting object remains I1; may be noted that the balanced type of de- Xed. As a practical matter, however, if there is tector circuit described above is desirable for the purpose of balancing out any amplitude modulation that may be present. It is described and claimed in the application of vIrving Wolff and Royden C. Sanders, Serial No. 445,010, filed May 29, 1942, and, entitled Frequency modulated a1- timeter or distance indicator. Any amplitude modulation that may be present in the mixing signal or in the reflected signal or any jamming signal is balanced out in the primary 22. rlhe beat frequency signal from tube I6 fand the beat frequency signal from tube I1 will add in the primary 22 and' will appear across secondary 23 Where it is impressed upon the audio amplifier 24. It should be understood that the invention is not limited to the use of this balanced type o-f receiver as any suitable receiver may be employed.

In accordance with the present invention, a regenerative feedback circuit is provided through a suitable delay network or other delay means 2l which preferably has a time delay equal to l/fm where fm is the frequency-modulation frequency. Various types of delay networks which are suitable for use in my invention are known in the art. Also, instead of electrical networks there may be employed delay devices of the acoustic type or of the magnetic recording type, for example. It will be apparent that if the beat-frequency signal fed back through the time delay device 21 is impressed upon the input circuit of audio amplifier 24 in phase with the signal being received from the detector lli-l1, there will be additive reinforcement of the desired beat frequency signal. There will beV substantially less reinforcement of random noise signals since this reinforcement willbe in accordance with the square root of thesum of the squares of such signals.

The operation of the invention may be seen more clearly by referring to Figs. 2 and 3. Fig. 2 illustrates the cyclic frequencymodulation of the transmitter where the period of the frequency sweep is l/fm. The beat-frequency signal that appears in the detector output circuit during each ofk these periodic frequency sweeps is shown in Fig. 3. It will be noted that, assuming no change in the distance between the transmitter and the reflecting object, the beat-frequency signal at the beginning of each periodic sweep starts at the same point in the cycle. Thus it is possible to delay the beat-frequency signal `by the Ytime l/Jm so that, at the audio amplifier input circuit, the beat-frequency signal produced during one frequency-modulation sweep will occur in phase with and reinforce the beat-frequency signal produced during the neXt occurring frequency-modulation sweep.

By utilizing the present invention, a substantial increase in signal-to-noise ratio may be btained. The amount of improvement in signalto-noise ratio can be expressed by the equation where K is the gain through the regenerative feedback circuit and has a value less than one, and where constant gain over the frequency band is assumed.

It should be understood that the cyclic frequency modulation need not be in accordance with a sawtooth wave form (shown in Fig. 2) but, if preferred, may be in accordance with other wave shapes such as a sine wave or a triangular wave, for example,

Figure 4 shows an embodiment of the invention in which the desired delay in the feed-back circuit is obtained by means of magnetic recor ling on a moving tape 3| of suitable magnetic material. The tape 3| may be carried by a wheel 32V of non-magnetic material which is driven by a motor 33.

The output signal of the audio amplifier 24 is recorded on the tape 3| by means of a magnetic recording element R. At the end of one-half a rotation of the tape 3l the same signal is taken orf the tape by means of a pickup unit P and is supplied to the input circuit of amplifier 24. After the tape passes the pickup unit P, it is wiped off or erased by suitable means such as an erasing unit E supplied with alternating current from a source 36.

One -way of making certain that the amount of delay in the feedback circuit always has the proper relation to the modulation cycle `is to obtain the frequency modulating signal from la moving tape 3l' on which the modulating signal is permanently recorded, and to rotate this tape synchronously with the time ldelay tape 3l. In the example shown, the tape 37 is carried by a non-magnetic wheel 38 which is mechanically coupled to the same shaft that is driving thedelay tape 3i. One cycle of the modulating signal is permanently recorded on each half of the tape 3l whereby there are two modulating cycles for each rotation of the wheel 33, The recorded signal on each half of the tape 3l may be of any desired wave form such as a sawtooth or a sine wave. The recorded signal is taken off the tape by a pickup unit M which supplies the modulating ignal to the modulator I2 through a switch 35.

it will be evident that When the time delay device and the frequency modulating signal source are coupled mechanically Vas described, there is always the correct time delay to make one beatfrequency cycle reinforce the next occurring beat-frequency cycle. In this specific embodiment of the invention, the motor 33 need not be of the synchronous type but in the embodiment described below it should be synchronous.

instead of obtaining the frequency modulation by means .of a permanent recording, the switch 35 may be moved to its other position to connect the frequency modulator l2 (a vibrating capacitor unit, for example) to the cycle power line. Since the magnetic delay'tape 3| is being driven by a synchronous motor, the delay period will always be synchronous with the frequency-modulation cycle. Y

I claim as my invention:

l. A distance measuring system comprising means for cyclically frequency modulating a wave at a periodic rate for fm, means for transmitting said modulated wave tol a reflectingobject, a beat-frequency detector, means for. supplying said wave to said detector after reflection from said object, meansl for supplying a portion of said wave directly from the transmitter to said detector whereby a cyclically recurring beatfrequency signal is obtained, an amplifier for said beat frequency signaLand a regenerative feedback circuit for said amplier, said feedback circuit including means for introducing a delay substantially equal to l/fm times a whole number.

2. The method of distance measuring comprising cyclically frequency modulating a Wave at a periodic rate of fm, transmitting said modulated wave to a reflecting object, heterodyning said wave after reflection from said object with a portion of said wave supplied directly from the point of transmission whereby a cyclically recurring beat-frequency signal is obtained, amplifying said beat-frequency signal, delaying a portion of said amplified signal by an amount substantially equal to l/Jm times a Iwhole number, and utilizing said delayed signal to regeneratively reinforce said ampliiication.

3. A distance measuring system comprising a transmitter for producing a wave that is cyclically frequency modulated at a periodic rate of fm and for transmitting said modulated wave to a reecting object, a beat-frequency detector, means for supplying said wave to said detector after reection from said object, means for supplying a portion of said wave directly from the transmitter to said detector whereby a cyclically recurring beat-frequency signal is obtained, an amplier for said beat frequency signal, a delay network which provides a delay substantially equal to l/fm times a whole number, and means for coupling the output circuit of said amplier to the input circuit of said amplier through said delay network to introduce regeneration.

4. A distance measuring system comprising means for cyclically frequency modulating a wave at a periodic rate of fm, means for transmitting said modulated wave to a reiiecting object, a beat-frequency detector, means for supplying said wave to said detector after reiiection from said object, means for supplying a portion of said wave directly from the transmitting means to said detector whereby a cyclically recurring ybeat-frequency signal is obtained, an amplier for said beat-frequency signal, a regenerative feedback circuit for said amplier, said feedback circuit including an electro-mechanical delay means for introducing a delay substantially equal to l/fm times a whole number, and means for maintaining synchronism between said delay means and said cyclic frequency modulation.

5. A distance measuring system comprising means for producing a carrier wave, a modulating wave producing means which includes an element having thereon a record of a modulating 'wave and a pickup unit for reproducing said modulating wave, means for cyclically frequency modulating said carrier wave by said reproduced modulating wave at a periodic rate of fm, means for transmitting said modulated wave to a refleeting object, a beat-frequency detector, means for supplying said modulated wave to said detector after reflection from said object, means for supplying a portion of Said modulated wave directly from the transmitting means to said cletector whereby a cyclically recurring beat-frequency signal is obtained, an ampliiier for said beat-frequency signal, a regenerative feedback circuit for said amplifier, said feedback circuit including an electro-mechanical delay means for introducing a delai7 substantially equal to l/fm times a whole number, and synchronizing means coupling said delay means and said modulating Wave producing device.

6. The invention according to claim 5 wherein said modulating wave producing means and said delay means each include a. movable element and wherein said movable elements are mechanically connected.

ROYDEN C. SANDERS, JR.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,206,903 Lane et a1 July 9, 1940 2,280,109 Varela Apr. 21, 1942 2,212,420 Harnett Aug. 20, 1940 

